416 



BRITISH GRAPTOLITES. 



Figs. 281 a- 



-d. — Monograptus galaensis, 

 Lap worth. 





region ; this tendency disappears entirely in the distal portion, where the thecal 

 are conspicuously everted and the apertural margin produced into a distinct 



denticle. The overlap increases from one-third 

 proximally to two thirds in the more mature 

 portion of the polypary. 



Affinities. — Monog. galaensis has characters 

 which connect it with some of the members of 

 the group of M. priodon, especially such forms 

 as M. Halli, but the plane of the apertural 

 margin is never really retroverted, even in the 

 proximal region, so that M. galaensis seems 

 connected more closely with the members of the 

 group of M. vomer inns, and from these its more 

 slender form and the characters of the proximal 

 thecas should be sufficient to render its sepa- 

 ration fairly easy. 

 a b c a Horizon ami Localities. — Gala-Tarannon 



a. Proximal end, showing sicula. Fragment Beds (zones of M. tumCUldtuS and M. Cris/)HS.) 



on same slab as PI. XLII, fig. lb. 



b. Distal thecae. Fragment on' same slab as S. Scotland. — Meigle Quarry ; Quarry near 



c. Distal theeEe in low relief. Meigle Quarry, Old CambuS, CockburilSpatll, 5 miles N. by B. 



S. Scotland; Gala Beds. Coll. Lapworth. pt i niii t-\ • i ni -i 



d. Fragment showing the long walls of the ot Lauder; Buckholm ; Darmck ; Glendearg; 



distal thecce. Meigle Quarry, S. Scotland ; -iti- i m i • i 



Gala Beds. Coll. Lapworth. WOOplaW ; I lisllielaw, etc. 



Wales. — Forge Corner, Conway. 

 Collections. — Geological Survey of Scotland, Lapworth, and the Authors. 







Gkoup IV. 



Monograpti in which the mature thecas are tubes with the apertural region 

 more or less isolate and retroverted. 



In the Monograpti included in this group, the ventral walls of the thecse 

 show ogee curvature, and the isolation and retroversion of the apertural region 

 give to the terminal portion of the theca the form of a more or less distinct hool\ 

 This hook, however, is never bent back upon itself, in other words the axis of the 

 theca in the course of its growth never bends through an angle exceeding 180°. 

 The proportion of the total breadth of the polypary occupied by the characteristic 

 hook varies considerably in different species. In some, the proportion of the 

 breadth may be as much as two-thirds (M. priodon), or it may be the merest 

 fraction (M. Halli). There is also considerable variation in the proportion of the 

 theca involved in the retroversion of the apertural region. 



There is commonly, if not invariably, some torsion of the thecal axis. As a 

 rule the torsion is in a positive direction, so that viewed from one side (the 



